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Narcolepsy is estimated to affect more than 200,000 Americans and only a fraction of them are actually diagnosed. This neurological disorder is the second-most common cause of excessive daytime sleepiness, after obstructive sleep apnea.

Though not necessarily a dangerous condition -- unless the sufferer is driving or operating machinery -- its symptoms can be embarrassing at the least or extremely devastating to personal and professional development and relationships.

There are four typical symptoms of narcolepsy: sleep attacks, cataplexy, sleep paralysis and hypnagogic (sleep-related) hallucinations.

When a person with narcolepsy falls asleep anytime or anywhere with literally no warning, it's called a sleep attack. Narcolepsy's best-known symptom, sleep attacks can happen multiple times a day and, though commonly lasting a few minutes, can be an hour or more in duration.

The person often feels refreshed when he wakes but can still fall asleep again within minutes.

Normal sleep in a healthy person begins with about an hour or so of NREM (non-rapid eye movement) sleep, during which the entire body slows down. Breathing, pulse, blood pressure and brain waves all decrease.

REM (rapid eye movement) sleep begins when the brain waves become very active, similar to those of the waking brain. This is the part of the sleep cycle where most dreaming occurs. A little while later, the sequence begins again.

Researchers have discovered that in a narcoleptic sleep attack, REM sleep begins almost instantaneously, with no preliminary NREM sleep.

Cataplexy, the second classic narcolepsy symptom, is a paralysis of sorts, in which the sufferer experiences a sudden, temporary episode of muscle weakness that is triggered by intense emotions such as laughter, anger, surprise or elation.

The person doesn't lose consciousness and usually only the arms and/or legs are affected. Ssometimes the head drops and the jaw goes slack and, in severe cases, the person may fall down and become completely paralyzed for a short time.

Just before falling asleep or while waking, a narcoleptic can experience the third classic symptom of narcolepsy: sleep paralysis. This causes a totally conscious person to be unable to speak or move. It usually lasts only a few seconds.

Many experts believe that, similar to cataplexy, sleep paralysis may be attributed to the fact that during REM sleep the brain is active but motor activity is inhibited. (Sleep paralysis is not only a characteristic of narcolepsy. It occurs in otherwise healthy people, too.)

Sleep-related hallucinations (hypnagogic phenomena) most often occur just before sleep and occasionally upon awakening. The person experiences dreamlike visual and/or auditory hallucinations (again an occurrence not reserved exclusively for narcoleptics).

Narcolepsy is difficult to diagnose because symptoms vary from person to person, and only a small percentage of narcoleptics have all four classic symptoms.

Though the disorder remains a mystery, it is known insomnia or sleep deprivation is almost never the cause, despite sleep apnea and other sleep disorders leading to striking daytime drowsiness. Only after specific studies are performed at a sleep disorder clinic can narcolepsy be definitively diagnosed.

Linda Napier , a registered nurse and independent patient advocate living in Southbury, is author of the book "Tender Medicine." You can send her e-mail at lindanapier@netzero.com.